1. Field of the Invention
The present invention relates to an organic EL (electro-luminescent) display device, and more particularly to a circuit provided to each pixel of an organic EL display device.
2. Description of the Related Art
The inventors of the present invention have developed a pixel circuit for driving an organic EL display device as described in U.S. Pat. No. 6,950,081 (patent document 1).
The inventors of the present invention also have developed a novel pixel circuit to which a technique described in patent document 1 is applied. The inventors of the present invention have further studied a layout of a pixel circuit which can decrease an area of a pixel. With respect to a conventional layout of a pixel, U.S. Pat. No. 6,998,770 (patent document 2), U.S. Pat. No. 6,522,079 (patent document 3) and U.S. Pat. No. 6,723,576 (patent document 4) are named.
In patent documents 2 to 4, there is disclosed a conventional pixel circuit which arranges two transistors and one capacitance in one pixel. This pixel circuit is one of most typical pixel circuits. A video signal is supplied to video signal lines. A selection signal is supplied to selection signal lines. The first transistor (data fetching TFT) in the pixel is controlled in response to a selection signal so as to fetch the video signal to the pixel. The capacitance holds the difference in potential corresponding to the video signal. The second transistor (drive TFT) is controlled corresponding to a magnitude of the difference in potential held by the capacitance so as to supply a desired electric current to an organic EL element from a power source line.
The planar layout of the pixel circuit is disclosed in patent document 2 or 3. There is disclosed the structure in which one power source line is arranged in a short-side boundary between two pixels, and the second transistor is provided on left and right sides of the power source line so as to supply an electric current to an organic EL element. That is, the second transistors are arranged with a planar layout in which the second transistors are arranged in symmetry in the lateral direction with respect to the power source line, and the power source line is used in common by left and right pixels. Due to such structure, the number of power source line is reduced and, at the same time, a voltage drop is reduced thus realizing a high-definition organic EL display device.
Further, patent document 4 also discloses a conventional pixel circuit which arranges two transistors and one capacitance in one pixel.
With the technique disclosed in patent document 2 and patent document 3, however, there has been a case in which the pixel cannot accommodate the pixel circuit therein so that the high definition of the pixel cannot be acquired, a case in which it is necessary to narrow a line width of the power source line thus giving rise to a gradient in brightness, or a case in which it is necessary to reduce the capacitance thus making the gray-scale expression less than optimal.
In FIG. 13 of patent document 4, there is disclosed a pixel layout in which pixel circuits of two upper and lower pixels are arranged in line symmetry, pixel circuits of two left and right pixels are also arranged in line symmetry so that neighboring four pixels to each other are arranged in point symmetry. However, none of these patent documents discloses an idea that a constitutional element (TFT or capacitance) which constitutes the pixel can be used in common by the neighboring pixels.